Machining the teeth of double sided face gears

ABSTRACT

On account of the constructional form of the tool head ( 17 ) according to the invention, the opposite sides ( 18, 19 ) of a tool ( 13 ) attached to the tool spindle ( 14 ) of a numerically controlled continuously generating gear grinding or hobbing machine are capable of machining the upper and lower sets of teeth ( 2, 3 ) of a double-sided face-gear ( 1 ) in one and the same set-up and without disturbing the synchronization between the rotations of the grinding worm and workpiece maintained during the grinding of the first set of teeth, without risk of collision. This eliminates the need to reset the workpiece ( 1 ) between the machining of the two sets of teeth ( 2, 3 ), thereby shortening the overall machining time substantially, and allowing the avoidance of accuracy losses due to the resetting of the workpiece ( 1 ).

[0001] The invention concerns a process and a device for the grinding or hobbing of the teeth of face-gears.

[0002] Thanks to the modern NC technology on present day machine tools, it has become possible to machine the teeth of crown-gears or face-gears to maximum precision by the continuous generating process. The decisive step to this end was the solving of the task of making the required geometrically very intricate worm-shaped tool to the necessary degree of accuracy.

[0003] High precision face-gears afford advantages in helicopter gears, amongst other things, because angular drive designs are then possible which would be impossible or at least far more difficult to construct with bevel gears. There are gear designs in existence, for example, which in spite of high transmitted power are of compact dimensions and light in weight, thanks to torque splitting. Torque splitting means that in the course of transmission to an output shaft the input torque is divided up and applied to the same driven gear via two or more tooth engagements. For a specified power, this measure allows the driven gear to be made smaller, and hence lighter.

[0004] An advantageous design for such a gear stage with torque splitting requires a driven face-gear which has two opposite sets of teeth on its periphery. Engaging with each of these sets of teeth is a pinion which transmits half of the total torque. It is of particular importance for the optimum exploitation of torque splitting that the division of the torque is very exact. With respect to their angular position about the axis of rotation, therefore, the two sets of face-gear teeth must be aligned very accurately one to the other.

[0005] The manufacture of such double-sided face-gears has as yet been very complicated, and must be performed in two operations:

[0006] 1. Machining of the first set of teeth; then removal, reversal and renewed setting up of the workpiece.

[0007] 2. Exact alignment of the already machined set of teeth relative to the machining tool, such that the second set of teeth is machined at the specified position relative to the first set; then machining of the second set of teeth.

[0008] This process is time consuming, and embodies the risk of accuracy loss.

[0009] It is an object of the invention to introduce a process and a device which considerably facilitate the manufacture of such double-sided face-gears, and which assure a higher attainable accuracy. This task is achieved by way of the features of the claims 1 and 3.

[0010] The invention is explained in the following by the example of grinding the two sets of teeth of a double-sided face-gear on a numerically controlled continuous generating face-gear grinding machine. It is however equally applicable in the same sense to a numerically controlled face-gear hobbing machine.

[0011] According to the invention the process consists in the use of an especially designed tool head that allows the worm-shaped tool to be engaged on two opposite sides to machine the two sets of teeth of a double-sided face-gear in one and the same set-up, without disturbing the synchronization maintained between the grinding worm rotation and workpiece rotation while grinding the first set of teeth.

[0012] A number of advantages are thereby gained simultaneously:

[0013] 1. The removal and resetting of the workpiece to grind the second set of teeth are dispensed with, which shortens the overall machining time substantially.

[0014] 2. The workpiece and work fixture need therefore only be designed for one set-up configuration.

[0015] 3. The two machined sets of teeth run very exactly concentric and in angular definition to each other, which results in an improved quality in load distribution.

[0016] 4. The mutual rotational alignment of the sets of teeth can be effected very easily and to high precision via the control system, which likewise contributes to an increase in the overall gearing quality.

[0017] A further special advantage of the process according to the invention is that it is possible to grind topologically corrected double-sided face-gears, where the topology is produced by modifications to the tool flank profile, one after the other without having to re-profile or exchange the tool between the two operations. This is due to the fact that during machining, the working and non-working flanks of both sets of teeth make contact with the same portions of the tool, whereas in the conventional method with resetting of the workpiece between the machining of the first and second sets of teeth the allocation of the tool flanks to the workpiece flanks alters.

[0018] A constructional example of the invention is illustrated with reference to the FIGS. 1 to 4. These depict:

[0019]FIG. 1 the axial cross-section through a double-sided face-gear,

[0020]FIG. 2 the working area of a conventional face-gear grinding machine,

[0021]FIG. 3 the device according to the invention, for grinding double-sided face-gears in one and the same set-up,

[0022]FIG. 4 diagrammatically the grinding process according to the invention, and

[0023]FIG. 5 a perspective of a sector of the teeth of the face-gear.

[0024]FIG. 1 shows diagrammatically the cross-section of a double-sided crown-gear or face-gear 1 such as is used for example in helicopter gears. The driving torque is transmitted in part to each of the two sets of teeth 2, 3 by the separate driving pinions 4, 5. The axes 6, 7 of the pinion shafts 8, 9 can be parallel or set at a mutual angle of inclination 5. In the second case at least one of the sets of face-gear teeth 2, 3 is concave or convex.

[0025] The above mentioned exact tangential alignment of the face-gear teeth 2, 3 one to the other can be such that the tooth space centre of both sets of teeth lie in on a defined radius in the same plane through the face-gear axis, or on the other hand mutually offset by the offset angle ε.

[0026]FIG. 2 shows the working area of a conventional NC-controlled continuously generating face-gear grinding machine. The face-gear 1 to be machined is set up by means of a suitable work fixture 10 on the work spindle 11. During machining it rotates about its axis 12. The grinding worm 13 is attached to the grinding spindle 14, which is located for rotation in the grinding head 15, and driven by the grinding motor 16 via a means 25 here only symbolically indicated. During the machining of the face-gear teeth 2, the workpiece 1 and grinding worm 13 rotate in mutual synchronization according to the ratio between the number of thread starts on the grinding worm and the number of teeth on the crown-wheel, whilst the grinding head 15—and hence also the grinding worm 13—are moved along the imaginary pinion axis 6, controlled by the NC axes X, Y and Z. After inverting the workpiece 1, the set of teeth 3 are machine in the same manner, the grinding worm 13 being moved along the imaginary pinion axis 7.

[0027]FIG. 3 depicts the device according to the invention for the grinding of double-sided face-gears in a single set-up. The grinding worm 13 is attached to the grinding spindle 14, which is located for rotation in the grinding head 17. Here the grinding head 17 is so designed and the driving motor 16 so arranged that the grinding worm 13 can engage with either of the two sets of face-gear teeth 2, 3 with two of its sides 18, 19 without colliding with the workpiece 1. The grinding head 17 projects in the direction X radial to the workpiece axis 12, and the bearings and driving components of the grinding spindle 14 are either narrower than the diameter of the grinding worm 13 in the Z-direction, or offset axially relative to the grinding worm 13 such that they cannot collide with the workpiece 1 and with the work spindle 11 either during the machining of the set of teeth 2 or during the machining of the set of teeth 3. To this purpose in the second case the bearings and driving components of the spindle 14 are offset in the axial direction of this spindle at least half the outside diameter of the workpiece 1 relative to the axial midpoint of the worm 13. Moreover the work fixture 10 has an axial extension 30, of which the outside diameter 31 is less than the difference between the inside diameter 32 of the set of teeth 3 and the outside diameter of the grinding worm 13. The axial length 33 of the extension 30 corresponds roughly to the difference between the diameter of the grinding worm 13 and the length of that part of the workpiece hub 34 projecting beyond the set of teeth 3 of the workpiece 1.

[0028]FIG. 4 depicts diagrammatically the process according to the invention for grinding the two sets of face-gear teeth 2, 3 of the face-gear 1 in a single set-up. In the example illustrated here the set of teeth 3 is machined first. Commencement could just as well be made by grinding the set of teeth 2. The starting point for the motion of the grinding worm 13 in the machine cycle is the position 20. The grinding worm is firstly moved in the known manner backwards and forwards on the paths 21 along the imaginary pinion axis 7 across the teeth 3 to be ground, and infed incrementally according to the machining allowance on the pre-machined face-gear teeth. On reaching the finished dimension of the set of teeth 3, the grinding worm 13 is retracted a distance 22 without disturbing the synchronization between tool and workpiece necessary for grinding, which distance 22 permits a subsequent non-colliding shift 23 to the starting position 24 for the grinding of the set of teeth 2.

[0029] Prior to the grinding procedure on the set of teeth 2, the grinding worm 13, which is still rotating synchronous with the workpiece, is shifted by a relative tool rotation angle of φ. The magnitude of this angle depends on the number of teeth on the face-gear, the number of thread starts on the grinding worm, the desired angle of offset ε between the two sets of face-gear teeth 2, 3 and the angle of inclination 6 of the two sets of teeth relative to each other; i.e. the deviation out of parallel of the imaginary pinion axes 6 and 7. The rotation of the grinding worm 13 through the tool rotary shift angle φ is effected to high precision via the NC control system, which calculates the exact angular magnitude with reference to the above stated input parameters. After rotating the grinding worm 13 through the tool rotary shift angle φ, the worm is once again moved in the manner described for the set of teeth 3 parallel to the imaginary pinion axis 6 to and from across the set of teeth 2, and infed according to the desired material removal rate until the finished dimension is attained. After the retraction of the grinding worm to the starting position the machining of the two sets of teeth 2, 3 has been terminated. 

1. Process for the continuous grinding or hobbing of the teeth of double-sided face-gears on a numerically controlled face-gear grinding or hobbing machine, wherein firstly the one set of teeth (3) is machined, then the tool (13) is aligned by means of the NC machine axes X, Y and Z with respect to its angular position and its location relative to the work piece (1) in the position required for machining the second set of teeth (2), without interrupting the synchronization between the rotations of the tool and workpiece (1) maintained while machining the first set of teeth (3), and then the second set of teeth (2) machined.
 2. Process according to claim 1, wherein for purpose of its rotational alignment with the angular position of the second set of teeth (2) after machining the first set of teeth (3) the tool (13) is rotated through an angular amount φ. $\phi = {\frac{ɛ \cdot z}{g} - {\frac{360}{2 \cdot g} \pm \delta}}$

relative to its theoretically desired position, which is defined by the synchronization with the workpiece rotation for machining the first set of teeth, where: φ=Tool rotational shift in degrees ε=Angular offset between the reference tooth spaces of the two sets of face-gear teeth (2, 3)in degrees g=Number of thread starts on the tool z=Number of teeth on the face-gear (2, 3) δ=Deviation off parallel between the pinion axes (6,7) in degrees.
 3. Device for the continuous generating grinding or hobbing of face-gear teeth on a numerically controlled continuous generating gear grinding or hobbing machine with a work spindle (11) and a workpiece (1) set up on the spindle by means of work fixture (10), and a tool head (17) movable relative to the workpiece (1) via NC-axes X, Y and Z together with tool spindle (14) and worm-shaped tool (13) attached to the same, the rotations of the workpiece (1) and the worm tool (13) being synchronized one to the other according to the number of teeth on the workpiece (1) and the number of thread starts on the worm tool (13), wherein due to the constructional form of the tool head (17), both the one zone (18) and the other opposite zone (19) on the circumference of the tool (13) located in bearings in it can be brought into machining engagement with the one set of teeth (3) or the opposite set of teeth (2) of the workpiece (1), without collision between the latter and the tool head (17).
 4. Device according to claim 3, wherein the tool head (17) is narrower, measured in the direction of the workpiece axis (12), than the outer diameter of the tool (13).
 5. Device according to claim 3, wherein the bearings and driving components of the tool spindle (14) in the tool head (17) are offset relative to the axial midpoint of the worm tool (13) by an amount at least half the outside diameter of the workpiece (1) in the direction of the tool spindle axis.
 6. Device according to claim 3, wherein the work fixture (10) is provided with an extension (30) in the direction of the work spindle axis (12), the outside diameter (31) of which said extension is less than the difference between the inside diameter (32) of the sets of teeth (2, 3) and the outside diameter of the worm tool (13). 